Embodiments of the present invention relate to a noise generator for generating a noise signal, and to a corresponding method.
Noise generators may, for example, be applied in channel simulators simulating the propagation effects and the impairment of signals by analog processing stages.
One of these disturbing influences to be simulated may, for example, be the so-called “phase noise” of oscillators. The simulation for phase noise is usually performed by generating noise characterized or colored in accordance with specifications by the user. This is conventionally obtained from white noise which is then filtered in accordance with specifications by the user. The result of the noise generated in this way may either be used directly as a noise signal and transferred to a phase signal or (advantageously) buffered. The buffered result is exemplarily loaded to an arbitrary generator, reproduced cyclically and applied to a phase modulator which in turn modulates the useful signal.
The technological challenge here is that the real phase noise spectrum is to be simulated exactly over a wide frequency range, like from 1 Hz to 10 MHz, for example, with a dynamics of, for example, 100 dB or more. Furthermore, the period of the signal is to last as long as possible before repeated again.
There are different ways of generating signals in accordance with such requirements, which will be discussed below. The colored noise may, for example, if applicable, be generated on a separate computer using suitably established IIR or FIR filters. When using IIR filters, it is of disadvantage that the estimation method for adjusting the filter coefficient to the frequency response used, due to the high dynamics, use high a filter order and may become numerically unstable. Consequently, the result has to be checked by an experienced user. However, this is not acceptable for an application in a measuring device.
When using FIR filters, the coefficients thereof can easily be estimated reliably. However, the number of coefficients used is very high (like 224). This consequently consumes much calculating time and memory when determining the coefficients and subsequently filtering the white noise. An interactive operation by the user and direct implementation in a measuring device can hardly be realized at present.
Irrespective of the filter type used, one problem is that the wide frequency range uses arbitrary generators with a relatively high sample rate (like 20 Msamples/sec) and, at the same time, high a storage depth (like 1000 seconds corresponding to 40 gigabytes) in order to obtain a sufficiently realistic simulation. The amount of time for calculating the noise sequence and loading the arbitrary generator consequently are in the range of minutes and, consequently, inacceptable for the user.
In order to (apparently) improve the repetition rate of the colored noise, a trick may be applied. Instead of generating a single noise signal, two noise signals are generated, the lengths of which are prime to each other. The two are reproduced and added in separate arbitrary generators before being applied to the phase modulator. In the frequency range where the signals of the two arbitrary generators overlap, however, there are frequently deviations from the desired frequency response. A very much longer period (like 207 days) seems to be achievable in this way. In case there are only two noise signals, however, the autocorrelation function of the sum signal is bad and the short repetition durations of the two arbitrary generators become visible.
Consequently, the object underlying the present invention is providing a noise generator for a broad frequency spectrum, which represents an improved compromise between operating convenience, resource efficiency (calculating time and memory requirements) and frequency response deviation.